1. Field of the Invention
The present invention relates to a method of producing a workpiece. More particularly, the present invention relates to a method which applies a dry pressing process to produce a workpiece with high hardness.
The present invention relates to a method of producing a workpiece; more particularly, the present invention relates to a method which applies a dry pressing process to produce a workpiece with high hardness.
2. Description of the Related Art
The dry pressing process is a common process in traditional powder metallurgy. In the dry pressing process, a powder is filled into the mold, and then pressure is applied to the powder to compress the loose powder and form a green part with a certain density. Finally, the green part is sintered to form a workpiece. The process can be used to automatically produce a net-shaped workpiece at low cost. Therefore, in machinery manufacturing, the dry pressing process is a necessary process.
Generally speaking, in the dry pressing process, for a workpiece to have desirable mechanical or physical properties, the density of the workpiece should be increased, which means the density of the green part should be increased to reduce the sintering temperature and time and thereby to reduce costs. Furthermore, after sintering, the shrinkage of the workpiece of a green part of high density is less than the shrinkage of a workpiece of a green part of low density. Therefore, the dimensional stability of a workpiece formed from a high-density green part is superior to the dimensional stability of a workpiece formed from a low-density green part. The major factors affecting the green part density are:
(1) Pressure of forming: In the dry pressing process, using higher pressure for forming produces a green part with a higher density. However, the metal powder is subject to work-hardening. Therefore, when the pressure increases, the hardness of the powder will also increase, such that the increasing efficiency of the green part density will gradually decrease with the increasing forming pressure. Furthermore, when the pressure of forming increases, the friction between the powder and the mold will increase, too. Therefore, the surface of the mold may be damaged.
(2) Powder feature: The hardness of the powder affects the density of the green part. A powder with a high hardness is not easily deformed, and thus the powder cannot easily be filled into the pores between the powders. Therefore, the green part density cannot be increased easily, and the workpiece cannot have a high density. The shape, the size, and the internal structure of the powder affect the forming of the powder. For example, the compressibility of a powder with an irregular shape and internal pores is poor, and the compressibility of a powder with a regular shape and no internal pores is good. In contrast, the friction of a powder with spherical shape is small, and the apparent density is high. Thus, the density of the green part will be high.
In addition to the powder shape and internal structure, the size of the powder affects the density of the green part. The contact area between fine powders is greater than the contact area between coarse powders. Thus, in the fine powder, the friction is great, and the apparent density is low. Therefore, the powder must be pressed with a greater forming pressure to obtain the required green part density. Furthermore, a fine powder does not flow easily, so the fine powder cannot be filled into the mold cavity via an automatic process. However, the sintering driving force of the fine powder is great, and the density of the workpiece of the fine powder is high.
Therefore, to produce a workpiece with high density, a fine powder and a high green density must be applied to increase the density of the sintered part. However, the fine powder must be pressed by a great pressure to increase the density of the green part, and the great pressure may cause the mold to be damaged. Furthermore, if the hardness of the powder applied in the dry pressing process is high, then the difficulty of the process will increase. Therefore, the dry pressing process manufacturer usually does not produce workpieces with high sintered density and high hardness. For example, if an alloy powder with a hardness of 320 HV(32 HRC) is applied in the dry pressing process, then the powder will not easily be deformed during the pressing process, the compressibility will be poor, and the density of the green part will be low. If a common size powder (one with a mean particle size higher than 44 μm) and a common pressure (400-800 MPa) are applied in the dry pressing process, the green density of the workpiece usually will be less than 6.3 g/cm3, or less than 80% of the theoretical density. Since the density of the green part is low, and since the mean particle size is large, the density of the workpiece and the mechanical properties will be low.
Therefore, there is a need to provide a new method to produce a workpiece by powder metallurgy. In the new method, via the dry pressing process, a workpiece with high hardness and great density can be produced, and the damage to the mold caused by the pressure of the pressing process can be reduced.